1. Field of the Invention
The present invention relates to a method of and an apparatus for copy controlling a coordinate measuring probe with a rotary table, and more specifically to a method of and an apparatus for copy controlling a coordinate measuring probe with a rotary table suitable for use in a three-dimensional coordinate measuring machine. The rotary table is settled on a platen of a coordinate measuring machine, and an object (workpiece) to be measured is placed on the rotary table and rotated by the same to copy measure the coordinates of the workpiece using the coordinate measuring probe. One axis by the rotary table is incorporated in the three-axis copy control of the probe when the rotary table is not used, that has already been realized, and copy operation by simultaneous four-axes control is assured keeping unchanged a direction of the probe with regard to an axis center of the object.
2. Description of the Prior Art
For measuring the shape of an object to be measured using a coordinate measuring machine such as a three-dimensional (3-D) coordinate measuring machine or the like, it is necessary to copy measure the coordinates of the object in succession with a coordinate measuring probe such as a touch signal probe and a copying probe, etc. For automated methods that automatically achieve such copy control utilizing a computer but without use of the rotary table, there have already been developed and realized two control methods: one where copy control is performed along the contour of a workpiece keeping the height of the probe constant from an arbitrarily assigned reference plane (hereinafter, referred to as constant height copying), and the other where copy control is performed along the contour of a workpiece in a cylindrical plane defined by an arbitrarily assigned radius from an arbitrarily assigned straight reference line (hereinafter, referred to as constant radius copying).
There is, however, found no copy control method with use of a rotary table, that satisfies necessary conditions.
As illustrated in FIG. 2, it is typical to set a rotary table 12 on a platen 10 of a coordinate measuring machine. parallely or perpendicularly to an axis X, Y or Z of the coordinate measuring machine. Such parallel or perpendicular installation of the rotary table is difficult, however, and care must be taken to ensure that the rotary table 12 can be mounted or demounted. Accordingly, conditions must be taken into consideration that the rotary table 12 should be placed on the platen 10 at an arbitrary position and with an arbitrary inclination independently of the axis of the coordinate measuring machine.
Additionally, the same can be said for an interrelation between an object 14 to be measured (a workpiece for example) placed on the rotary table 12 and the axis of rotation of the rotary table 12. More specifically, although it is common to place the workpiece 14 on the rotary table 12 such that a measuring reference line of the workpiece 14 is coincident with the axis of rotation of the rotary table 12, it is preferable to allow the workpiece 14 to be placid on the rotary table 12 at an arbitrary position and with an arbitrary inclination.
It is imagined that any lack of the foregoing two conditions would bring about very severe restriction to actual use of the apparatus.
Further, for a given positional relationship between the probe 16 and the workpiece 14, when the probe 16 is performing constant radius, copying, the position of the probe is measured with respect to a given radius from an imaginary measuring reference line (work axis, for example). However, this operating configuration allows innumerable positional relations as illustrated in FIG. 3. More specifically, any of the probes 16A, 16B, and 16C shown in FIG. 3 is located on a plane spaced away a predetermined distance from the work axis so that there are existent innumerable combinations of probe positions with respect to a certain rotation angle .theta. of the rotary table 12 under the restriction only of the distance from the work axis. Therefore, data processing becomes very complicated.
The rotary table 12 is preferably used to position the workpiece 14 so that only tip of the probe 16 (and not the body of the probe 16) makes contact with the workpiece 14. Preferably, a line connecting the measuring ball of the probe 16 to the measuring reference line of the work intersects the reference line at a constant angle even when the Table 12 (i.e. the workpiece 14) rotates as shown in FIG. 4. FIG. 4 is a plan view illustrating the situation of the rotary table 12 viewed from the just above portion of the rotation axis of the rotary table 12 when the rotation axis of the rotary table 12 and the work axis measuring reference line are parallel to each other. In the figure, the angle of the probe 16 relative to the rotation axis of the rotary table 12 is maintained constant at all times.
To sum up, the copy control method and apparatus using a rotary table should satisfy the following three conditions:
No. 1. The rotary table can be set on the platen of the coordinate measuring machine at an arbitrary position and at an arbitrary angle.
No. 2. A workpiece can be mounted on the rotary table at an arbitrary position and an arbitrary angle.
No. 3. The angle of the probe should be maintained constant during a copy measurement.
There is, however, found no rotary table copy control method that satisfies such conditions, until now.